There is an increased demand for electrical components which can protect electronic circuits from EOS transients which produce high electric fields and usually high peak powers capable of destroying circuits or the highly sensitive electrical components in the circuits, rendering the circuits and the components non-functional, either temporarily or permanently. The EOS transient can include transient voltage or current conditions capable of interrupting circuit operation or destroying the circuit outright. Particularly, EOS transients may arise, for example, from an electromagnetic pulse, an electrostatic discharge, lightening, or be induced by the operation of other electronic or electrical components. Such transients may rise to their maximum amplitudes in microsecond to subnanosecond time frame and may be repetitive in nature.
Materials for the protection against EOS transients (EOS materials) are designed to respond essentially instantaneously (i.e., ideally before the transient wave reaches its peak) to reduce the transmitted voltage to a much lower value and clamp the voltage at the lower value for the duration of the EOS transient. EOS materials are characterized by high electrical resistance values at low or normal operating voltages and currents. In response to an EOS transient, the material switches essentially instantaneously to a low electrical resistance value. That is, EOS materials have a non-linear resistance as a function of voltage. When the EOS threat has been mitigated these materials return to their high resistance value. These materials are capable of repeated switching between the high and low resistance states, allowing circuit protection against multiple EOS events. EOS materials are also capable of recovering essentially instantaneously to their original high resistance value upon termination of the EOS transient. For purposes of this application materials which exhibit a non-linear resistance as a function of voltage will be referred to as "voltage variable" materials
FIG. 1 illustrates a typical electrical resistance versus d.c. voltage relationship for EOS materials. Circuit components including EOS materials can shunt a portion of the excessive voltage or current due to the EOS transient to ground, thus, protecting the electrical circuit and its components. The major portion of the threat transient is reflected back towards the source of the threat. The reflected wave is either attenuated by the source, radiated away, or re-directed back to the surge protection device which responds with each return pulse until the threat energy is reduced to safe levels.
Typical EOS materials comprise a polymeric binder with conductive, semiconductive and insulative particles dispersed therein. Examples of prior EOS polymer composite materials are also disclosed in U.S. Pat. Nos. 4,331,948, 4,726,991, 4,977,357, 4,992,333, 5,142,263, 5,189,387, 5,294,374, 5,476,714, 5,669,381, and 5,781,395.
In a typical prior application of polymeric EOS materials, the material is placed in a gap formed by two confronting electrodes disposed on a supporting substrate. The edges of the electrodes form the active electrode area. An example of such an edge-to-edge electrode configuration is disclosed in International Publication Number WO 97/26665. This edge-to-edge electrode configuration has several drawbacks. First, while the overall electrode may occupy a relatively large planar area, the active area of the electrode (i.e., the portion of the electrode in contact with the EOS material) is relatively small. Moreover, in order to obtain the proper clamping voltages for certain applications the gap between the edges of the electrodes must be extremely narrow, e.g., two (2) mils. It is often difficult to control the manufacturing process to (1) form such a precise, narrow gap and (2) deposit the EOS material in such a small space. The present invention is provided to solve these and other problems.